Combining seismic waves with seismoelectrics to perform prospecting and measurements

ABSTRACT

A method and apparatus for low cost, convenient prospecting and surveys of subsurface structures using seismoelectric signals, as well as laboratory analysis of geological samples. The seismoelectric signals come from seismic waves generated by sources which can be applied or natural. The seismoelectric signals are generated with the same velocity and frequency of the generating seismic waves, then induce secondary electromagnetic signals which travel at their own much higher speed. The seismoelectric signals thus may be measured with electrodes or antennas. Electrodes may be disposed within a borehole or on the surface. The method allows use of geophone data, but does not require it. The it source of the seismoelectric signals, being a moving seismic wave front, conveys continuous, whole body information on the structures underground, in much the same way as seismic ware data, but in the form of simpler, easier to capture seismoelectric signals. Reflection and refraction of seismic waves can be ascertained. The method also can be used to determine permeability and water table level. The measurement does not require use of signal phase, nor geophones nor multi-chambered pressure oscillators.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of Provisional PatentApplication Serial Number 60/078,020 filed Mar. 13, 1998.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for prospecting andgeophysical surveying using seismoelectric signals. This can be used fora geophysical survey, either at the surface, above the surface, in aborehole, or in the laboratory. One feature of the invention is that theseismic wave source may be artificial or natural. The method of theinvention then analyzes the seismoelectric signal utilizing therelationship that the seismoelectric signal's source travels withseismic wave velocity and frequency and carries information aboutunderground formations. The invention can thus be used for survey ofunderground formations and also, in particular for of permeability.Alone or when combined with the seismic wave data, seismoelectricsignals provide a more detailed and less costly geophysical survey.

2. Description of Related Art

The traditional seismic survey uses geophones, essentially verysensitive geophones placed in contact with the earth's surface or thewalls of boreholes in order to capture seismic signals (vibrations inthe medium) which are generated by any of a variety of sources, natural,and artificial. These surveys have a number of disadvantages:complexity, cost, and the fact that the geophones must receive a certainthreshold value of seismic signal in order to be activated and recordthe signal.

Low frequency electric signals have been known to propagate through theEarth's crust for some time. The sources of these signals are numerous.Geotelluric signals are caused by geological reactions within the Earth.The telluric signal is caused by the solar wind, a steady flow ofelectrically charged particles emanating from the sun. Seismoelectricsignals are caused by the interaction of a geological matrix such aspermeable earth material, and water within it, under the effect of aseismic disturbance. Other causes of low frequency subterraneanelectrical signals are also known.

These various types of signals have been hopefully examined by surveyorsfor decades, without any useful theoretical basis for scientificanalysis of the results.

In the realm of more specific and scientifically verifiable knownmethods, there are several patents which teach use of streamingpotential in measurement of permeability of geological samples.

Streaming potential is the electrical potential (voltage) generated bywater flowing in a solid matrix such as permeable rock. The oppositeeffect, electro-osmosis, is the generation of water flow or waterpressure in a permeable matrix by application of an electrical potentialacross the matrix.

U.S. Pat. No. 3,599,085 teaches use of a sonic transducer periodicallyexciting a formation (matrix) at low frequencies to cause periodicelectrokinetic potentials which are measured at a location near to thetransducer and at a location spaced from the transducer, the ratio ofthe measured potential being related to the electrokinetic skin depth toprovide an indication of the permeability of the formation. U.S. Pat.No. 4,427,944 teaches application of pressure of alternating polarity tothe matrix and measurement of the generated transient streamingpotential in the time domain to estimate the characteristic responsetime of the matrix. U.S. Pat. No. 5 4,742,402 teaches the building of aseismoelectric signal recording device. Finally, U.S. Pat. No. 5,417,104and its continuation, U.S. Pat. No. 5,503,001, teach determination ofpermeability of porous media and thickness of mudcake on the walls ofboreholes and thin porous media by measuring at a finite frequencystreaming potential induced in such thin layers by finite frequencypressure oscillations. This method uses multi-chambered apparatus,requires that the electrodes all be quite close to the source of thepressure oscillations, and requires measurement of the extremely smallphase shift between the components of the streaming potential signal.

These methods also appear to lack data handling methodology based on anunderstanding of the nature of the propagation of seismoelectricsignals, as follows. The streaming potential creates a seismoelectricsignal, energized by the passage of a seismic wave. A secondaryelectromagnetic field is induced by the seismoelectric signal, which canbe detected in its proximity. The source of the seismoelectric signal isthe passage of the seismic wave, and the seismoelectric signal sourcethus betrays the location, velocity and frequency of the seismic wave.Since the known methods do not teach that seismoelectric signals arepropagated at the velocity and frequency of the seismic wave, they alsodo not point to convenient, low cost methods for using seismoelectricsignals for surveys in a wide variety of environments: boreholes, on thesurface, above the surface, or in the lab. Furthermore, seismoelectricsignals are related not only to permeability and resistivity, but alsoto seismic wave forms dependant upon the P-wave (compression) and S-wave(shear) velocities. As a result, the known art methods are limited tospecial cases such as lab work, limited geological formations ordetermination of the thickness of mudcake in a borehole.

SUMMARY OF THE INVENTION

It is one object of this invention to overcome many of the disadvantagesof known streaming potential measurement methods.

It is another object of this invention to provide a method ofgeophysical surveying which does not require a threshold value ofseismic signal in order for the seismic signal to be detectable.

It is another object of this invention to provide a method of use of therelationship between seismic waves and seismoelectric signals inseismoelectric surface prospecting.

It is another object of this invention to provide a method of use of therelationship between seismic waves and seismoelectric signals instreaming potential measurements inside a borehole to evaluatepermeability.

It is another object of this invention to provide a method to detectthree dimensional seismic wave signals using an aerial antenna insteadof geophones.

It is another object of this invention to provide a simple, low costmethod for conducting accurate and quantifiable geophysical boreholesurveys and geophysical borehole prospecting.

It is another object of this invention to provide a simple, low costmethod for conducting accurate and quantifiable geophysical aerialsurveys and geophysical aerial prospecting.

It is another object of this invention to provide a simple, low costmethod for conducting accurate and quantifiable laboratory testing ofgeological samples.

It is yet another object of this invention to provide a low cost methodfor accurate, quantifiable surveying without use of expensive equipment.

It is yet another object of this invention to provide simple, low costapparatus for conducting accurate and quantifiable geophysical surfacesurveys and geophysical surface prospecting.

It is yet another object of this invention to provide simple, low costapparatus for conducting accurate and quantifiable geophysical aerialsurveys and geophysical aerial prospecting.

It is yet another object of this invention to provide a method to carryout seismoelectric prospecting using natural seismic sources.

It is yet another object of this invention to provide simple, low costapparatus for conducting accurate and quantifiable laboratory testing ofgeophysical samples.

It is yet another object of this invention to provide simple, low costapparatus for locating a subterranean water table.

In general, the method of the invention uses the nature of propagationof seismic waves and the generation of seismoelectric signals by thosewaves to map subsurface features and permeability data. As a seismicwave travels through a water permeated subsurface matrix of earthmaterials, it generates seismoelectric signals. Previous researchers inthe field did not realize that the data received showed that theseismoelectric signal was being generated at the same velocity andfrequency as the seismic wave generating it. The seismoelectric signalthen radiates (at the considerably greater velocity of a subterraneanelectromagnetic signal) away from that point of generation, while theseismic wave continues to generate new seismoelectric signals as ittravels through the subsurface terrain. In the method of the invention,these seismoelectric signals are captured in the time and frequencydomains by at least one pair of electrodes. The information they capturecan then be analyzed using the method of the invention to tell thevelocity and frequency of the original seismic wave. The propagation ofseismic waves is relatively well understood, and the velocity andfrequency data from that wave can be used to determine the subsurfacetopology, using methods well known in the art.

Compared to older, traditional, geophone surveying, the method ischeaper, more convenient, uses less complex A machinery, and, since theseismoelectric signals follow Ohm's law, is very sensitive, not having aminimum threshold value for detection.

BRIEF DESCRIPTION OF THE FIGURES

The above objects and advantages of this invention will become furtherapparent upon reading the detailed description of the preferredembodiment and alternative embodiments of the invention, with referenceto the drawings. In brief:

FIG. 1 is a diagram showing the apparatus for a pendulum seismoelectricanalysis in the laboratory.

FIG. 2 is a graph showing the data generated by the apparatus of FIG. 1.

FIG. 3 is a graph showing an attenuating sine wave.

FIG. 4 is a graph showing the data generated by the apparatus of FIG. 1superimposed onto the attenuating sine wave of FIG. 2.

FIG. 5 is a diagram of the apparatus of the preferred embodiment of theinvention for use in a seismoelectric signal surface survey, showing theequipotential and penetrability whole body volumes yielded by aseismoelectric survey.

FIG. 6 is a diagram of apparatus of one alternative embodiment of theinvention for conducting a seismoelectric survey in a borehole.

FIG. 7 is a two dimensional viscoelastic model of a geological formationhaving three layers.

FIG. 8 is a graph of a U-component of a seismic wave model whose sourceis located in the top left of FIG. 7.

FIG. 9 is the graph of a W-component of a seismic wave model whosesource is located in the top left of FIG. 7.

FIG. 10 is a graph of data from a seismoelectric signal model generatedby a seismic signal source located in the top of the second layer ofFIG. 7, measured between two electrodes of FIG. 1, spaced 0.1 km apart.

FIG. 11 is a graph of data from a seismoelectric signal model generatedby a seismic signal source located in the top of the second layer ofFIG. 7, measured between two electrodes of FIG. 1, spaced 0.5 km apart.

FIG. 12 is a diagram showing seismic signals emanating from a naturalunderground source and through the Earth's crust, generatingseismoelectric signals at the water table which in turn induce anelectromagnetic field captured by an antenna in another alternativeembodiment of the invention.

FIG. 13 is a diagram showing seismic waves emanating from a naturalunderground source and reflecting within the subsurface structuresbefore being captured by seismoelectric receivers in another alternativeembodiment of the invention.

FIG. 14 is a graph of a U component, two dimensional seismic wave modelof data taken from the three layer model in FIG. 7, using the apparatusof FIG. 5 in a surface survey using a natural seismic wave sourcelocated at the bottom of the model.

FIG. 15 is a graph of a W component, two dimensional seismic wave modelof data taken from the three layer model in FIG. 7, using the apparatusof FIG. 5 in a surface survey using a natural seismic wave sourcelocated at the bottom of the model.

FIG. 16 is a graph of data from a seismoelectric signal model measuredbetween two electrodes of the preferred embodiment of the invention fromFIG. 5 in a surface survey per FIG. 14 and FIG. 15.

FIG. 17 is a graph of data from a seismoelectric signal model measuredbetween two more widely separated electrodes of the preferred embodimentof the invention from FIG. 5 in a surface survey per FIG. 14 and FIG.15.

FIG. 18 is a diagram showing seismic signals emanating from anunderground seismic source and producing seismoelectric signals whichproduce a secondary electromagnetic field in turn captured byseismoelectric receivers in another alternative embodiment of theinvention used to determine the depth of the water table.

DESCRIPTION OF THE INVENTION

The first consideration for a practical application of seismoelectricsis the nature of the propagation of seismoelectric signals. Aseismoelectric signal is the streaming potential produced by thepropagation of seismic waves inside a water-saturated medium such asearth materials. A seismoelectric signal exhibits the velocity andfrequency of the propagating seismic wave.

The source of a seismic signal can be a single controlled explosion, asis often used in known geophysical surveying methods, or an impact oranother source. After such an event, a dynamic field is set up. Insidethis dynamic field, a seismic wave is propagated from the source in alldirections. The propagation of the seismic wave inside a water-saturatedearth material causes a relative displacement of water and rock, andthis tiny relative displacement is the source of the seismoelectricsignal. The source of the seismoelectric signal is moving along with theseismic waveform as it spreads out from the seismic source; thereforethey have the same velocity. The principle that seismoelectric signalpropagation corresponds with seismic wave velocity can be understood bycomparison with the phenomenon of headlight traces of cars running alonga highway. The car is the seismic wave, the light is the seismoelectricsignal it generates. One could plot the position of the car by plottingthe position across time of the light source, the headlights. One canplot the position of the seismic wave by plotting the position acrosstime of the seismoelectric signal source. After the seismoelectricsignal is produced, it induces a secondary electromagnetic field. Thesource of the seismoelectric signals, the seismic wave, is traveling ata relatively slow speed of around 2 km/second, which is naturally farless than the secondary electromagnetic field speed. It is the speed ofthe seismic wave which is of interest in prospecting.

Well known techniques used over many years allow researchers todetermine the structures below the surface of the Earth based upon themotions of seismic waves. This invention provides a better method ofdetermining seismic wave motions.

In more detail, the theory is as follows.

In a water-saturated matrix, the differing physical properties of thewater and the solid matrix result in the creation of electrical doublelayers. The surface layer of the solid material naturally acquires onecharge, while the surface layer of the water naturally acquires theopposite polarity of electrical charge. In the water, this first chargedlayer is held in immobile contact with the solid material. This immobilelayer has been defined in the art as being the compact or inner regionvery near the wall of the matrix, in which the charge and potentialdistribution are determined chiefly by the geometrical restrictions ofion and molecule size and the short range interactions between ions, thewall surface and the adjoining dipoles, and layers of water further fromthe surface. This layer ends up essentially attached to the wall surfaceand its properties end up being similar to those of the solid matrix. Asthe distance from the surface increases, the charge in the waterdecreases. This zone of decreasing charge is called the diffuse layer.Momentum is conserved normally between these layers as seismic wavespass through them, the product of mass and velocity of each layer(momentum) being necessarily equal to that of the next layer. Differingmasses of rock and water then require differing velocities in rock andwater in order to conserve the momentum, and the streaming potential isgenerated by the pressure caused by the relative velocities. Theresulting sources of seismoelectric signals are subject to all the samealterations as usually occur to seismic waves: reflection, refractionand so on. Note also, the water table will cause a difference in theseismoelectric signals as it is crossed, allowing easy location of thewater table.

When a seismic wave travels in water-saturated earth materials, thereare three movements at the particle level that relate to theseismoelectric signals: rock movement, water movement and electriccharge movement. In a seismic wave field, rock movement causes watermovement. Relative movement between water and rock disturbs theelectrical double layers which in turn produce electric charge movement.Thus, the seismoelectric signal results from the movement of theelectric charges. Rock movement is the source of both water movement andelectric charge movement. Therefore seismoelectric signals velocity andfrequency depend directly on the seismic wave.

In addition, there are tiny phase shifts between rock displacement,water displacement, and the resulting streaming potential, these areknown and used in the prior art but use and calculation of these tinyquantities is not necessary to practice the present invention.

In the laboratory, the seismoelectric theory is tested by suspending acylindrical rock sample by two electrode wires, so that it formed apendulum. The force from the rock is coupled through the double layersto the water. The coupling from the rock to the water follows theconservation of momentum. Due to the different density between rock andwater there is a pressure difference between rock and water calledequivalent pressure. As the pendulum rock sample is swung, conservationof momentum generates the pressure difference between rock and waterwhich generates the streaming potential. The streaming potential signalthen becomes a function of both time and two dimensional space, andgenerates an electromagnetic trace which matches in velocity andfrequency the pendulum motion: an attenuated sine curve.

The equivalent pressure acting on the water inside of a rock sampledepends on the density difference of the water and the rock sample. Ifthe pendulum movement has high frequency or high velocity, the pressuredifference variations on the amplitude and frequency are greater thanthe limited frequency values found in known art. The water fluid insideof a porosity in such a case will be turbulent rather than laminar,although this lies outside the scope of this patent.

In the conceptual model, the pendulum motion becomes one particle'sdisplacement inside of the wave field of a seismic wave. The streamingpotential becomes a function of time and 3-D space, and shows itsseismoelectric signal character by traveling with the seismic wavevelocity and frequency.

Seismic waves propagate in solids as patterns of particle deformationtraveling through the material, with velocity dependent on the elasticproperties and densities of the material. In a compression or P-wave,the motion of the particles is always in the direction of wavepropagation. In a shear or S-wave, the motion of individual particles isalways perpendicular to the direction of wave propagation.

In a seismic wave field, when waves propagate inside of the watersaturated rocks, the force from rocks coupling to the water producesequivalent pressure, and this equivalent pressure producesseismoelectric signals.

A seismoelectric signal, the streaming potential, may contain usefulinformation regarding geophysical properties, including permeability.

Field surveys verify the laboratory analysis and the conceptual model.Analysis of the results of previous surveys reveals that in fact, thisconceptual relationship between seismoelectric signals and seismicsignals has been hinted at, without any awareness of this fact byresearchers. Based upon this knowledge, it is now possible to invent amethod and suitable apparatus to engage in geophysical surveying andprospecting using seismoelectric signals instead of or in conjunctionwith seismic waves, with various benefits accruing as previouslymentioned. Mathematical underpinnings for the method can now be derived.

Unlike a seismic wave, which can be detected by a geophone in onelocation, seismoelectric signals are detected using two electrodes andmeasuring the potential (voltage) across them.

Placement of the two electrodes is important. If one of the electrodesis placed far from the seismic source, and the other electrode inside ofthe seismic field, the inside electrode will reflect the point ofelectric potential change that corresponds to what a seismic geophonedetects in terms of velocity and frequency. On the other hand, during apractical survey, if we put one electrode too far from the seismicsource, the seismoelectric signal will be too weak to be measured.

It is preferable to place two electrodes inside of the seismic field. Inthis situation, the movements detected are different from those detectedby a seismic geophone. While the seismic geophone detects movement onlyat the point where it is located, the two electrodes reflect the wholebody movement where the whole body is assumed to be a half sphere with adiameter of the distance between the two electrodes. The streamingpotential is measured at each time during the interval while the seismicwave travels from the first electrode to the second. During this time,the seismic wave is penetrating to successively deeper layers of thesubsurface as the whole body hemisphere expands. Applying the Helmholtzseismoelectric formula to the whole body effect, and expressing it interms of stress in the rock, yields:

E(t)=(ω²Δρ(P 1(t)−P 2(t))∈ζ)/(4πησ  Eq. 1

In which E is streaming potential, ω is the frequency of the seismicwave, Δρ is the density difference between the rock and the water, P1and P2 are rock stresses of the seismic wave at electrodes 1 and 2, ∈ isthe permittivity of dielectric of the fluid, ζ is the zeta potential, ηis the viscosity and σ is the fluid conductivity.

The term “low frequency” means frequencies under about 200 Hertz. For alow frequency seismic wave, the longer the finite distance between theelectrodes, the deeper the penetration of the seismic wave which isgenerating the seismoelectric signals. This applies to both surfacesurveys and borehole surveys.

The force on a unit area of a rock cylinder, expressed as pressure P,produces a rock displacement u. According to Hook's law, these arerelated by the Lame constants. This displacement disturbs the doublelayer of electrical charges. The rock displaces the immobile layer inthe water, which in turn displaces the diffuse layer. The streamingpotential comes from this displacement, equivalent to the pressure ofthe water following the Helmholtz equation.

Two dimensional seismoelectric signal propagation equations inaccordance with the stress-strain relation (Voigt's solid formula) canthen be developed from the Helmholtz equation:

E(x12)=(ω²Δρ∈ζ/(4πησ))[∈_(xx)(e1)+∈_(zz)(e1)+∂∈_(xx)(e1)/∂t +∂∈_(zz)(e1)/∂t−∈ _(xx)(e2)−∈_(zz)(e2)−∂∈_(xx)(e2)/∂t−∂∈ _(zz)(e2)/∂t]  Eq.2

E(z12)=(ω²Δρ∈ζ/(4πησ))[∈_(zz)(e1)+∈_(xx)(e1)+∂∈_(zz)(e1)/∂t +∂∈_(xx)(e1)/∂t−∈ _(zz)(e2)−∈_(xx)(e2)−∂∈_(zz)(e2)/∂t−∂∈ _(xx)(e2)/∂t]  Eq.3

In which E(x12) is the streaming potential along the x axis betweenelectrodes 1 and 2 and E(z12) is the streaming potential along the zaxis between the same electrodes. In addition, ∈_(xx) (e1) is thedisplacement in the x direction along the x axis at electrode 1, ∈_(xx)(e2) is the displacement in the x direction along the x axis atelectrode 2, ∈_(zz) (e1) is the displacement in the z direction alongthe z axis at electrode 1 and ∈_(zz) (e2) is the displacement in the zdirection along the z axis at electrode 2. Partial derivatives of thedisplacement with respect to time are included, as are the samevariables used in Equation 1. Use of these equations allows us toanalyze a streaming potential signal in respect to time and arrive atthe motions of the seismic wave which generated the streaming potentialseismoelectric signal. Note that these equations are given in the twodimensional version, but the equivalent sets of equations for threedimensions can obviously be derived. It is also possible to reduce themback to the one dimensional versions and use that for some surveying.

Since the coupling of the layers can be done either using a sphericalmodel or a cylindrical model, the velocity can be analyzed in terms ofboth the compression wave (P-wave) and the shear wave (S-wave) of theseismic wave.

The water table is a further important facet of seismoelectricprospecting. When a seismic wave propagates through the geologicalsubsurface structures, the different impedance of the different layerswill cause multi-reflections among them, as pointed out above. Inparticular, however, when the seismic wave travels to the surface of theearth, it will produce a seismoelectric signal at the water table.

When the water table is near the surface of the Earth, thisseismoelectric signal can be captured with an antenna, eliminating theneed for geophones fixed in solid contact with the earth. “Near” isdefined to be no more than 5 meters using present day sensingtechnology.

When the water table is not near the surface but rather is below thesurface, the antenna can be combined with geophone surveys of natural orartificial, seismic sources to determine the depth of the water table.The seismic wave will generate a seismoelectric signal as it crosses thewater table. The difference in speed between the seismic wave travelingat its relatively slow speed and captured by a geophone, and theseismically induced electromagnetic field traveling at its much higherspeed and captured by the antenna provides the depth of the water table,This is a second technique for use of the seismoelectric signal, againdependant upon the method of analyzing seismoelectric signals as beinggenerated by the wave front of a seismic wave.

An alternative embodiment of the invention makes a three dimensionalseismic survey taken at or above the surface with an antenna. Regardlessof whether the seismic source is natural or artificial, around a watertable there will be a seismoelectric effect that produces anelectromagnetic signal which will, upon encountering the ground surface,propagate into the new medium of the atmosphere, just as it propagatedinto new layers of the crust. These signals can be captured with anantenna. Compared to the traditional method of surveying with ageophone, this method is greatly advantageous. First, a geophonecollects only the seismic data at a single point, but as pointed outearlier, the antenna, like an electrode, gathers the seismoelectricsignal. Second, a three component geophone array must be used to capturethree dimensional seismic waves, which three recordings must then becompared in order to deduce the three dimensional structure of theseismic signal. The seismoelectric signal, radiating from the whole bodysurface of the hemisphere being traversed by seismic waves, shows theentire three dimensional data in a single pickup.

In addition, an antenna can be made much more portable than a geophone.

Apparatus for laboratory testing of the propagation of seismic waves andseismoelectric signals is shown in FIG. 1. Sample 102 is suspended byelectrode 104 and electrode 106 from support 107, thus making the entiresample into a pendulum. Amplifier 108 receives and amplifiesseismoelectric signals and passes them on to data acquisition device110, which may be a computer, a tape recorder or other equivalents.

FIG. 2 shows the data collected during the oscillation of sample 102.FIG. 3 shows an attenuating sine wave of the motion of a pendulum. FIG.4 shows the data from FIG. 2, collected by oscillating sample 102, andthe attenuating sine wave, superimposed. The curves match, showing thatthe seismoelectric signal is in fact due to the motion of the sample,and shares the frequency of the sample motion.

Apparatus for surface surveying or prospecting using the method of thepreferred embodiment of the invention is shown in FIG. 5. Seismic source502 located on ground surface 504 produces seismic waves 506, 508, 510,and 512. Electrode 514 is positioned at seismic source 502, electrodes516, 518, 520 and 522 are positioned, respectively, further away fromseismic source 502. Not shown are data acquisition devices for capturingthe steaming potential between each of the electrodes 516, 518, 520 and522 and the seismic source electrode 514.

In practice, seismic source 502 produces seismic waves 506, 508, 510 and512. These propagate, into the subsurface in roughly hemispherical form,slowing down and speeding up depending upon subterranean composition atany given point, and being reflected from certain types of subterraneanfeatures. As the waves spread out, they cause seismoelectric signals(not pictured) which radiate from every part of the moving seismic wavefront. The location of the seismoelectric signals sources propagateswith the moving seismic wave fronts, with the velocity and frequency ofthe seismic wave fronts. The seismoelectric signals themselves actuallytravel at their own quite high velocity. The array of electrodes 516,518, 520 and 522 can map the location of the sources of theseismoelectric signals, as well as capturing frequency information, bothof which are actually based upon the propagation of the triggeringseismic waves.

Penetrability lines 524, 526 and 528 mark the furthest extent of depthpenetration and reflection for, respectively, electrodes 516, 518, and520. Note that each electrode does receive signals from each section ofwave front of each of seismic waves 506, 508, 510, and 512, but thediffering relative timing (dependant upon the speed of the seismic wavesin the various materials) is illustrated by the penetrability lines 524,526 and 528. Combining all the data gained yields the whole body datafor the subterranean topology. The electrodes 516, 518, 520 and 522 areplaced at a finite distance from the seismic source 502 and the seismicsource electrode 514, such that they can detect the streaming potentialgenerated by seismic waves originating at the seismic source 502. Thegreater the spacing of the electrodes, the deeper the penetration intothe surface.

FIG. 6 illustrates the configuration for a survey, oriented in avertical direction down a borehole, in a first alternative embodiment ofthe invention. Seismic source 602 produces seismic waves (not shown)which propagate through the earth materials matrix at their owncomparatively low velocity. Electrodes 604, 606, 608, 610, and 612 arepositioned so as to capture seismoelectric signals (not shown) triggeredby the seismic waves as they pass through water saturated earthmaterials. Lines of penetrability 614, 616, 618, and 620 again mark theextent of depth penetration for each of electrodes 606, 608, 610, and612 respectively. The greater the spacing of the electrodes, the deeperthe penetrability from the wall of the borehole into the rock formation.Not shown are data acquisition devices for capturing the streamingpotential in the time domain between each of the electrodes 606, 608,610 and 612 and the seismic source electrode 604.

Seismic source 502 and seismic source 602 may be any of a variety ofdevices well known to those skilled in the art: explosives, noisemakers, impact devices such as are often mounted on surveying trucks,and so on. Fabrication and use of electrodes 514, 516, 518, 520, 522,604, 608, 610, and 612 is also well known to those skilled in the art.

EXAMPLE I

The apparatus shown in FIG. 5, the preferred embodiment of theinvention, is used to model a surface survey. In the model, theapparatus is altered by having a geophone positioned with eachelectrode. A two dimensional, three layer, viscoelastic medium model isused, as shown in FIG. 7, in which Vp1=2.0, Vp2=3.0, Vp3=8.0 where Vp isthe velocity of compressional waves (P-wave) in each of the layers.Vs1=1.4, Vs2=1.8, Vs3=5.6, where Vs is the velocity of shear waves(S-waves) in each of the layers. Finally, pm1=0.1, pm2=0.2 and pm3=0.1mD, where pm is permeability of each of the layers. Viscoelasticparameters are Vvp=0.01 and Vvs=0.01, for the P-waves and S-waves, inthe top and bottom layers, and Vvp=Vvs=0.02 for the middle layer. Layersare numbered from the top down. Viscosity is 1.0, surface conductivityis 0.10 mhos M, and the dielectric permittivity of the liquid is aconstant (theoretically 8.85 times 10 the 12th power F/m.) Zetapotential waves with permeability.

The artificial seismic source is located near the top left of the surveymodel. To simplify the problem, tube waves are ignored. Results areshown in FIG. 8 through FIG. 11. FIG. 8 shows the U component (vertical)of the seismic signals, FIG. 9 the W component (horizontal) of theseismic signals. For the seismic signal, the wave fronts shown in thenecessarily small FIG. 8 and FIG. 9 are 0.012 in the seismic signal and0.06 second in the seismoelectric signals.

For FIG. 10 and FIG. 11 the spacing of the electrodes is increased, asthe propagation theory of seismoelectric signals shows that measuringdepth depends upon electrode spacing. In a practical surface survey,penetration depth will depend upon electrode spacing. In a practicalborehole well log, invasion zone surveying is one important method ofcharacterizing the target layer. Thus by using different electrodespacing, the different depths of the layers can be used to gain moreinformation about the invasion zone or the area surveyed. The finitedistances in this example are 160 meters between electrodes 514 and 520,200 meters between electrodes 514 and 522.

For clarity, the seismoelectric signals are amplified by a gain of fourtimes in amplitude. In the surface survey, the surface seismic wave ismuch stronger than the waves produced by reflections and refractionscaused by subsurface topology, and the seismoelectric signals mirrorthis effect faithfully. The seismoelectric signals produced by seismicwaves can be separated as to origin in surface, reflected or refractedwaves based on their different velocities, much as seismic waves alwaysare, as is known to those skilled in the art. The net result, however,is the acquisition of further information about subsurface features.

FIG. 10 is a graph which shows the seismoelectric signal (the streamingpotential) measured between electrodes 514 and 520, 160 meters apart.The attenuating sine wave of the seismic signal from FIG. 8 and FIG. 9is clearly visible in the seismoelectric signal. FIG. 11 graphs theseismoelectric signal measured between electrodes 514 and 522, 200meters apart. Again, the attenuating sine wave of the seismic signal isclearly visible. Thus, we see that the seismic signal is paralleled bythe much easier to measure seismoelectric signal.

The reflection information on the seismic wave as it crosses theboundaries of different layers is thus captured with considerable easein the form of seismoelectric signals. In addition, the seismoelectricsignals contain permeability information.

The simple traces shown in FIG. 10 and FIG. 11 can then be translated toobtain the parameters of the various layers, in a simple process inverseto that used to generate the graphs shown. This inverse process is usedfor traditional geophone surveys.

FIG. 12 shows an alternative embodiment of the invention in whichnatural seismic source 1202 is used in place of an imposed seismicsource. Ground surface 1204 is closely underlaid by water table 1206.Seismic signal 1208 propagates through the subsurface, and uponencountering water table 1206, it is propagated upwards asseismoelectric signals 1210, which can be captured with antenna 1212.This method allows capture of the whole body data and yet requires nofixed geophones, and even eliminates the need for electrodes as used inother embodiments. The practicality of capturing seismoelectric signalsin areas where water table 1206 and ground surface 1204 are quite closehas been known in the art, however, a method of use of theseseismoelectric signals for mapping of seismic signals, and thus forprospecting, has not been known.

EXAMPLE II

As mentioned above, natural seismic sources include the earth'srotation, earthquakes, tidal movements and others. All these movementsproduce seismic waves and the seismic waves in turn produceseismoelectric signals.

A natural seismic source is analytically equivalent to an imposed sourcewhich is located at the bottom of the model.

As shown in FIG. 13, a second alternative embodiment of the invention,natural seismic source 1302 is located at the bottom of a theoreticalterrain having ground surface 1304, water table 1306, first geologicalboundary 1308 and second geological boundary 1310, and geologicalanomaly 1311, which is the “target” of interest in the survey. Naturalseismic source 1302 emits seismic signals 1312, 1314 and 1316, which areshown reflecting and refracting as they progress towards ground surface1304, where the signals are captured by reference receiver 1318 and byfirst receiver 1320 and second receiver 1322.

First receiver 1320, second receiver 1322, and reference receiver 1318can be geophones directly picking up the seismic signal, as is known inthe art, or in accordance with the invention they may be electrodescapturing seismoelectric signals radiated by the seismic signals as theypropagate through the Earth's crust, or they may be both.

Reference receiver 1318 provides a control signal, showing thegeotelluric signal, the telluric signal from the solar wind, and othersources of electrical signals which must be canceled out for accuratesurveying. The signals from reference receiver 1318 can be removed fromthe signals from first receiver 1320 and second receiver 1322 to derivea time and amplitude plot of the effect of geological anomaly 1311 uponseismic signals 1314 and 1316.

This use of a natural seismic source is modeled, as per the previousexample I, using the same model given in FIG. 7 and the apparatus ofFIG. 5, without seismic source 502. Instead, the natural seismic sourceis equivalent to an artificial seismic source located at the bottommiddle of the geological model.

FIG. 14 is a graph of the U component of seismic waves gathered by thesurface survey apparatus used in the previous example, but without anyartificial, imposed, seismic source.

FIG. 15 is a graph of the W component of seismic waves gathered by thesurface survey apparatus used in the previous example, but without anyartificial, imposed, seismic source.

FIG. 16 is a graph of the seismoelectric signal measured betweenelectrodes 514 and 516 used in this example II, and respectivelycorresponding to reference receiver 1318 and first receiver 1320 of FIG.13, showing that it again closely corresponding to the signals from theseismic waves on the previous two graphs.

FIG. 17 is a graph of the seismoelectric signal measured betweenelectrodes 513 and 518 used in this example II and respectivelycorresponding to reference 1318 and second receiver 1322 if FIG. 13,showing that it also closely corresponds to the signal from the seismicwaves in the previous two graphs.

The seismic geophones and seismoelectric electrodes are placed on thesurface as in example I. Once again, interpretation of the signalsgathered is conducted according to numerical methods: the seismoelectricsignals are used to calculate the subsurface geology, the inverse of themodeling process.

EXAMPLE III

When the water table is not near to the surface of the earth, combiningthe seismoelectric signal and the seismic wave information willdetermine the depth of the water table, using either a natural orartificial seismoelectric source.

FIG. 18 shows this method. Natural seismic source 1802 is located underground surface 1804, and at an undetermined level in between is watertable 1806. Seismic wave 1808 propagates upwards through water table1806, generating a seismoelectric signal which in turn generateselectromagnetic signal 1810, which continues into the atmosphere whereit is captured by antenna 1812. Seismic wave 1808 is captured by seismicgeophone 1814.

The seismoelectric signal induces a secondary electromagnetic fieldwhich will travel with a velocity circa 300,000 km/s, while the seismicsignal will travel with a velocity circa 1 km/s. Note that both of thesevelocities depend upon the characteristics of the local subsurfacestructures. The time difference between the arrival of the two signalsthen provides a simple method for calculating the depth of the watertable.

The above invention has been disclosed in both preferred and alternativeembodiments in order to enable one skilled in the art to practice it.While numerous details have been set forth for illustrative purposes, itwill be obvious to those skilled in the art that the invention issusceptible to many equivalents, substitutions, and alterations withoutdeparting from the essential spirit of the invention. Nothing in theforegoing disclosure is to be taken to limit in any way the scope of theinvention, which is to be understood only on the basis of the followingclaims.

What is claimed is:
 1. A method of borehole surveying of geologicalstructures in water saturated media, comprising the steps of, generatinga low frequency seismic wave, measuring streaming potential in the timedomain between a seismic source electrode operatively connected to thecrust of the Earth at the location of the seismic wave generation and atleast one receiving electrode operatively connected to the crust of theEarth at a finite distance from the location of the seismic wavegeneration, wherein said at least one receiving electrode is disposeddown such borehole, and analyzing said steaming potential to determinemultidimensional propagation of said seismic wave with respect to time.2. A method of laboratory analysis of water saturated geologicalsamples, comprising the steps of: suspending a geophysical sample so asto form a pendulum, attaching a plurality of electrodes to saidgeophysical sample, inducing a oscillation in said geophysical sample,so as to induce streaming potential in said geophysical sample,measuring in the time domain streaming potential between said pluralityof electrodes, and calculating from said streaming potential vibratorymotions within said geophysical sample.
 3. Apparatus for boreholesurveying of underground geological structures comprising: a seismicsource, for generating a low frequency seismic wave, at least oneseismoelectric signal receiver comprising a seismic source electrodeoperatively connected to the crust of the Earth at the location of saidseismic wave source and at least one receiving electrode operativelyconnected to the crust of the Earth at a finite distance from saidseismic wave source, wherein said at least one receiving electrode isdisposed within such borehole, and at least one potentiometer,operatively connected to said seismoelectric signal receiver, forcapturing streaming potential generated by said seismic wave'smultidimensional propagation in the time domain.
 4. The apparatusaccording to claim 3, wherein said seismic source and said seismicelectrode are disposed within said borehole.
 5. An improved method ofsurveying using geophones in regions having water saturated media, inwhich the improvement comprises the steps of: operatively connecting aplurality of electrodes to the Earth's crust, while taking readings fromsaid geophones, measuring streaming potential generated by themultidimensional propagation of a seismic wave in the time domainbetween said plurality of electrodes, and analyzing said streamingpotential to determine multidimensional seismic propagation and partialderivatives of seismic propagation with respect to time.
 6. A method forlaboratory analysis of water saturated geophysical samples, comprisingthe steps of: attaching a plurality of electrodes to said geophysicalsample, inducing an oscillation in said geophysical sample, so as toinduce streaming potential in said geophysical sample, measuring in thetime domain streaming potential between said plurality of electrodes,and calculating from said streaming potential vibrations within saidgeophysical sample.
 7. An improved method of surveying in regions havingwater saturated media, in which the improvement comprises the steps of:measuring in an aerial antenna streaming potential in the time domain,and analyzing said streaming potential to determine multidimensionalseismic propagation with respect to time.
 8. An improved method ofsurveying in regions having water saturated media, in which theimprovement comprises determination of permeability by the steps of:placing an antenna at the survey location, measuring said antennastreaming potential in the time domain, and analyzing said streamingpotential as a function of rock stress to determine permeability ofgeological formations.
 9. An improved method for geological surveying inwater saturated media, comprising the steps of: measuring in the timedomain streaming potential generated by multidimensional propagation ofa seismic wave, analyzing said streaming potential as a function of rockstress to determine therefrom multidimensional seismic wave propagation.10. The improved method of claim 8 wherein the step of analyzing saidstreaming potential further comprises analyzing streaming potential inthe time domain as a function of rock stress to determine therefrompermeability.
 11. The improved method of claim 10, wherein the functionof streaming potential in the time domain to rock stress takes the form:E(t)=f(ω, ρ, P(t), ζ, ∈, η, σ), wherein E(t) is streaming potential withrespect to time, f is the function, ω is the frequency of said seismicwave, ρ is the density difference between the water and said media, P(t)the rock stresses with respect to time, ζ the zeta potential, ∈ thepermittivity of the fluid, η the viscosity of the fluid, and σ the fluidconductivity.
 12. The method of multidimensional surveying of geologicalstructures in water saturated media of claim 9, wherein the function ofstreaming potential in the time domain to rock stress takes the form:E(t)=f(ω, ρ, P(t), ζ∈, η, σ), wherein E(t) is said streaming potentialwith respect to time, f is said function, ω is the frequency of saidseismic wave, ρ is the density difference between the water and saidmedia, P(t) the rock stresses with respect to time, ζ the zetapotential, ∈ the permittivity of the fluid, η the viscosity of thefluid, and σ the fluid conductivity.
 13. A method of surveying ofgeological structures in water saturated media, comprising the steps of:generating a low frequency seismsic wave, measuring streaming potentialin the time domain, and analyzing said streaming potential in the timedomain as a function of rock stress to determine therefrommultidimensional seismic wave motions.
 14. The method ofmultidimensional surveying of geological structures in water saturatedmedia of claim 13, wherein the function of streaming potential in thetime domain to rock stress takes the form: E(t)=f(ω, ρ, P(t), ζ, ∈, η,σ), wherein E(t) is streaming potential with respect to time, f is thefunction, ω is the frequency of said seismic wave, ρ is the densitydifference between the water and said media, P(t) the rock stresses withrespect to time, ζ the zeta potential, ∈ the permittivity of the fluid,η the viscosity of the fluid, and σ the fluid conductivity. 15.Apparatus for surveying of underground geological structures comprising:a seismic source, for generating a low frequency seismic wave, at leastone antenna, and at least one potentiometer, operatively connected tosaid antenna, for capturing streaming potential generated by saidseismic wave's multidimensional propagation in the time domain.
 16. Theapparatus of claim 15, wherein said antenna comprises an aerial antenna.